The present invention relates to, in a lithography process for transferring a predetermined pattern to a substrate coated with a photosensitive material through a projection optical system, an aligning method, an exposure apparatus using this aligning method, and a semiconductor device manufacturing method utilizing this exposure apparatus.
The lithography process in the manufacture of a semiconductor device uses an exposure apparatus for transferring a circuit pattern formed on a reticle or mask (to be referred to as a reticle hereinafter) to a wafer or glass plate (to be referred to as a substrate hereinafter) coated with a photosensitive material (to be referred to as a resist hereinafter). In this exposure apparatus, it is very important to perform alignment of the reticle and substrate relative to each other, i.e., so-called alignment, at high precision.
An alignment flow in a conventional exposure apparatus will be described with reference to FIG. 7.
First, prealignment is performed (step 71). Then, the positions of alignment marks formed on a plurality of sample shots set in advance from all shots are sequentially measured (step 72). The results of position measurement are statistically processed to calculate all shot arrangements (step 73). The respective shots are exposed on the basis of the calculation results (step 74).
In position measurement of the alignment mark, the alignment mark formed on the substrate is illuminated through or not through a projection lens, and light reflected and diffracted by the alignment mark is received by a light-receiving means through or not through a projection lens. Position information is obtained from information obtained by the light-receiving means. As the light used to illuminate the alignment mark (to be referred to as alignment light hereinafter), non-exposure light with a wavelength different from that of light used for exposure (to be referred to as exposure light hereinafter) is used because of absorption, photosensitivity, and the like of the resist applied to the substrate.
This alignment can be performed in two methods, i.e., a method that does not employ a projection lens (to be referred to as an off-axis method hereinafter) and a method that employs a projection lens (to be referred to as a TTL method hereinafter). The alignment mark can be detected in two methods, i.e., a method of forming an image of the alignment mark on an image sensing element and observing it directly (to be referred to as an image method hereinafter) and a method of using a grating-like mark as the alignment mark to detect a spatial phase (to be referred to as a phase detection method hereinafter). Each of these alignment methods has its merits and demerits, and generalization as to which method is better cannot be made.
These alignment methods will be described.
According to the off-axis method, since the alignment light does not pass through a projection lens, alignment is not adversely affected by the optical characteristics of the projection lens. Thus, the wavelength of the alignment light can be set freely, and accordingly, an optical system used for alignment (to be referred to as an alignment optical system hereinafter) can be designed freely. That is, the off-axis method can cope with various different processes.
In the off-axis method, due to the spatial design limit of the alignment optical system and projection lens, the alignment position and the exposure position are largely different from each other. After alignment is ended, the substrate stage on which the substrate is loaded is largely driven to the exposure position. At this time, if the distance between the alignment position and exposure position (to be referred to as a base line hereinafter) is always stable, no problem occurs. In fact, however, the base line changes over time and is not stable due to the influence of the ambient atmosphere of the exposure apparatus and the like. Hence, to stabilize the base line, measurement and correction must be performed at a predetermined time interval. This in turn decreases the throughput by the time spent for measurement and correction of the base line. Also, since the off-axis method is performed not through a projection lens, alignment does not follow the behavior of the projection lens.
According to the TTL method, the alignment light passes through the projection lens, which is advantageous in terms of stability of the base line and the follow-up performance to the behavior of the projection lens.
Since the projection lens is designed such that its aberration becomes optimal to the wavelength of the exposure light, the aberration with respect to alignment light with a wavelength different from that of the exposure light undesirably becomes large. For this reason, in Japanese Patent No. 2,633,028, when alignment is performed in accordance with the TTL method by using alignment light with a wavelength different from that of exposure light, a correction optical system is provided for correcting aberration produced by the projection lens, and alignment is performed through this correction optical system.
According to the image method, the alignment mark is illuminated through the alignment optical system. Light reflected by the alignment mark forms an image on the image sensing element through the alignment optical system. The position of the formed image is read to obtain position information. The alignment optical system may or may not include a projection lens, that is, it can employ the off-axis method or the TTL method.
When alignment is performed by using the TTL method and image method, the following problems arise. For example, when a KrF excimer laser beam (wavelength: 248 nm) is used as the exposure light, as the glass material that forms the projection lens is limited to quartz, fluorite, or the like due to the transmittance required and the like, the aberration of the projection lens with respect to non-exposure light becomes very large. It is difficult to design a correction optical system that corrects this large aberration, and a large numerical aperture (to be referred to as NA hereinafter) required by the alignment optical system employing the image method cannot be obtained. To remove this problem, a correction optical system may be provided in the projection lens to correct the aberration. In this case, however, the correction optical system affects not only the alignment light but also the exposure light.
In Japanese Patent Laid-Open No. 5-343291, the phase detection method is employed in place of the image method, that is, the TTL method and phase detection method are used, thereby solving the problem concerning aberration. According to this reference, as shown in FIG. 5, a grating-like mark 16 as an alignment mark is illuminated with alignment light 13, and xc2x1n-order diffracted beams 14 (n is a natural number) produced by the grating-like mark 16 are brought to interference with each other through a spatial filter 15. The spatial phases of interference fringes 17 formed by the interference are detected, thereby performing alignment.
When the phase detection method is used in this manner, the NA required by the alignment optical system of the phase detection method can be decreased more than that in the alignment optical system of the image method. For example, assume that a KrF excimer laser beam is used as the exposure light, an HeNe laser beam (wavelength: 633 nm) is used as the alignment light, an alignment mark with a grating pitch of 10 xcexcm is used, and only xc2x11-order diffracted beams are brought to interference with each other through a spatial filter to form interference fringes. If the angle at which the xc2x11-order diffracted beams emerge is defined as xcex8, since sin xcex8=0.063, the minimum necessary NA for the alignment optical system is 0.063. In practice, since the beam spot diameters of the diffracted beams must be considered, the minimum necessary NA for the alignment optical system is approximately 0.08. In this manner, alignment can be done by an alignment optical system with a small NA.
Therefore, since aberration only needs to be corrected within a small NA range of 0.08 or less, a case wherein the KrF excimer laser beam is used as the exposure light can also be coped with.
However, in the phase detection system, process factors such as nonuniform resist coating, nonuniform grinding in the CMP process, and the like may disturb the shape of the alignment mark, thereby degrading the alignment precision. The degradation of the alignment precision in the phase detection method is larger than that in the image method.
As described above with reference to FIG. 7, in the conventional exposure apparatus, alignment is performed in accordance with a flow in which prealignment is performed and after that the positions of alignment marks formed on a plurality of sample shots set in advance from all shots are sequentially measured. Hence, when position measurement is to be performed with the sample shots, the measurement positions of the alignment marks vary by an amount corresponding to the prealignment precision.
If the shapes of the alignment marks are not adversely affected by the process factors and are not disturbed, when the positions of the alignment marks are to be measured, even if the measurement positions of the alignment marks vary, the alignment measurement values do not vary, and alignment can be performed at high precision. On the other hand, if the shapes of the alignment marks are disturbed by process factors and the symmetry is lost, when the measurement positions of the alignment marks vary, the alignment measurement values also vary, degrading the alignment precision.
In the above description, degradation in alignment precision is described as a problem concerning the phase detection method. The same problem arises in the image method as well, although the seriousness of the problem may differ.
The present invention has been made in view of the problems of the prior art, and has as its object to provide an aligning method, exposure apparatus, and semiconductor device manufacturing method in which the alignment precision is not degraded even when the shape of an alignment mark is disturbed by process factors.
In order to achieve the above object, according to the present invention, there is provided an aligning method of measuring a position of an alignment mark within a plurality of sample shots formed on a substrate with a predetermined measurement condition, and positioning the substrate on the basis of a position measurement result, wherein the measurement condition is selected on the basis of a change amount of a position measurement value which is obtained from the position measurement result obtained by moving the alignment mark in a direction different from a measurement direction and sequentially measuring a position of the alignment mark.
According to the present invention, there is provided an exposure apparatus for measuring a position of an alignment mark within a plurality of sample shots formed on a substrate with a predetermined measurement condition, positioning the substrate on the basis of a position measurement result, and transferring a predetermined pattern onto the substrate through a projection lens, the exposure apparatus comprising means for moving the alignment mark in a direction different from a measurement direction, sequentially measuring a position of the alignment mark, and obtaining a change amount of a position measurement value from the position measurement result.
A semiconductor device manufacturing method according to the present invention utilizes this exposure apparatus.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.